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A common octopus with a forked arm that was found in south Crete, Greece
All cephalopods possess flexible limbs extending from their heads and surrounding their beaks. These appendages, which function as muscular hydrostats, have been variously termed arms, legs or tentacles.[A]
In the scientific literature, a cephalopod arm is often treated as distinct from a tentacle, though the terms are sometimes used interchangeably, often with the latter acting as an umbrella term for cephalopod limbs. Generally, arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends. Barring a few exceptions, octopuses have eight arms and no tentacles, while squid and cuttlefish have eight arms (or two "legs" and six "arms") and two tentacles. The limbs of nautiluses, which number around 90 and lack suckers altogether, are called tentacles.
The tentacles of Decapodiformes are thought to be derived from the fourth arm pair of the ancestral coleoid, but the term arms IV is used to refer to the subsequent, ventral arm pair in modern animals (which is evolutionarily the fifth arm pair).
The males of most cephalopods develop a specialised arm for sperm delivery, the hectocotylus.
Cephalopod limbs bear numerous suckers along their ventral surface as in octopus, squid and cuttlefish arms, or in clusters at the ends of the tentacles, as in squid and cuttlefish. Each sucker is usually circular and bowl-like and has two distinct parts: an outer shallow cavity called an infundibulum and a central hollow cavity called an acetabulum. Both of these structures are thick muscles, and are covered with a chitinous cuticle to make a protective surface. Suckers are used for grasping substratum, catching prey and for locomotion. When a sucker attaches itself to an object, the infundibulum mainly provides adhesion while the central acetabulum is free. Sequential muscle contraction of the infundibulum and acetabulum causes attachment and detachment.
^A study has determined that the octopus has two legs and six arms, often commonly referred to as "tentacles".  Another study found that there is a functional difference in the way the appendages are used for task division. "These findings give evidence for limb-specialization in an animal whose 8 arms were believed to be equipotential." The two rear appendages are generally used to walk on the sea floor, while the other six are used to forage for food.
^Thomas, David (12 August 2008). "Octopuses have two legs and six arms". The Telegraph. Retrieved 2018. To most of us it has always seemed obvious that an octopus has eight arms. Octopuses have two legs and six arms Claire Little, a marine expert from the Weymouth Sea Life Centre in Dorset, said: 'We've found that octopuses effectively have six arms and two legs.'
^ abNorman, M. 2000. Cephalopods: A World Guide. ConchBooks, Hackenheim. p. 15. "There is some confusion around the terms arms versus tentacles. The numerous limbs of nautiluses are called tentacles. The ring of eight limbs around the mouth in cuttlefish, squids and octopuses are called arms. Cuttlefish and squid also have a pair of specialised limbs attached between the bases of the third and fourth arm pairs [...]. These are known as feeding tentacles and are used to shoot out and grab prey."
^Fukuda, Y. 1987. Histology of the long digital tentacles. In: W.B. Saunders & N.H. Landman (eds.) Nautilus: The Biology and Paleobiology of a Living Fossil. Springer Netherlands. pp. 249-256. doi:10.1007/978-90-481-3299-7_17
^Toll R.B., Binger L.C. (1991). "Arm anomalies: cases of supernumerary development and bilateral agenesis of arm pairs in Octopoda (Mollusca, Cephalopoda)". Zoomorphology. 110 (6): 313-316. doi:10.1007/BF01668021.
^Okada Y.K. (1937). "An occurrence of branched arms in the decapod cephalopod, Sepia esculenta Hoyle". Annotated Zoology of Japan. 17: 93-94.
^Bradbury H.E., Aldrich F.A. (1971). "The occurrence of morphological abnormalities in the oegopsid squid Illex illecebrosus (Lesueur, 1821)". Canadian Journal of Zoology. 49 (3): 377-379. doi:10.1139/z71-055.
^Voss G.L. (1957). "Observations on abnormal growth of the arms and tentacles in the squid genus Rossia". The Quarterly Journal of the Florida Academy of Sciences. 20 (2): 129-132.